I run an 8.5 outdoors/big track - so as I guessed you're over-motored - and I am currently running on a track inside a gymnasium, in which the layout is larger than a basketball court (don't know how it compares to yours), and found even my 10.5 was too much motor (have since gone to a 13.5 and it's still more than enough, turned down to 90%). I've never run mini-spikes, compared to mini-pins, but it would seem that they'd slip more and wear even faster. We only just moved indoors for the winter, so I don't know how many race nights I'll get out of a set of tyres, but 15 runs seems to be totally reasonable, IMHO.
One other thing to consider, and not knowing what set-up you're running, is thinking about taking out some of the rear toe-in - being as the added traction isn't needed on carpet. While not an option, I'm guessing you could get away with 0* rear toe, indoors on carpet, compared to the normal 3-4* that's pretty much standard outdoors.
There is a member here "OneKiwi," from Sweden, that is selling some VBC brand slicks - so maybe that's an option you can consider.
http://www.oople.com/forums/showthread.php?t=84024